Mechanical component and method of surface hardening

ABSTRACT

Mechanical component having a surface at least one part of which has been surface hardened by induction heating. A cross section of the mechanical component through the surface exhibits a hardness H surface  at the surface, and remains substantially equal through a first region, a hardness H core  at the non-hardened core of the mechanical component and remains substantially equal through a third region and a transition, second region spanning between the first and third regions. The hardness profile in the first region has an average hardness Y 1 , and the hardness profile in the third region has an average hardness Y 3 . If a line is drawn on the hardness profile in the second region between the points (formula) and (formula), where 0&lt;K&lt;2, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage application claiming the benefit of International Application Number PCT/SE2011/000094 filed on 27 May 2011, which claims the benefit of SE Application 1000719-3 Filed on 2 Jul. 2010.

TECHNICAL FIELD

The present invention concerns a mechanical component and a method for surface hardening at least one part of a surface of such a mechanical component.

BACKGROUND OF THE INVENTION

Induction hardening is a heat treatment in which a metal component is heated to the ferrite/austenite transformation temperature or higher by induction heating and then quenched. The quenched metal undergoes a martensitic transformation, increasing the hardness and brittleness of the surface of a metal component. Induction hardening may be used to selectively harden areas of a mechanical component without affecting the properties of the component as a whole.

U.S. Pat. No. 4,949,758 discloses a process for hardening the interior surface of a long (8-32 feet i.e. 244-975 cm), thin-walled (wall thickness ⅛ to ¼ inch, i.e. 3-6 mm), small inside diameter (1¼to 3¼ inch, i.e. 32 to 83 mm) tubular member. More particularly, it relates to a process involving progressive heating with an internally positioned, electromagnetic induction coil, followed by immediate quenching with a quench ring assembly, to develop a martensitic case on the inner surface of the tube. This method is used to obtain a surface having a hardness of at least 58 HRC and a substantially non-hardened core with a sharp demarcation between the hardened surface and the non-hardened case core.

When surface hardening a surface of a component it is however advantageous to obtain a hardness profile that does not exhibit a sharp demarcation between the hardened surface and the non-hardened core of the component. A smooth demarcation between the hardened surface and the non-hardened core, i.e. a transitional region in which the hardness decreases steadily with depth rather than abruptly minimizes or eliminates any stresses in the material in that region when the component is in use. Such a steadily decreasing hardness profile may be obtained by carburizing the surface of the component.

Carburizing is a heat treatment process in which iron or steel is heated in the presence of another material that liberates carbon as it decomposes. The surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the surface becomes hard, while the core remains soft (i.e. ductile) and tough.

SUMMARY OF THE INVENTION

An object of the invention is to provide an improved non-through hardened mechanical component.

This object is achieved by a mechanical component having a flat or non-flat surface, i.e. an interior or exterior surface, at least one part of which has been surface hardened by induction heating. The surface namely comprises a martensitic microstructure produced by induction hardening using an electromagnetic induction coil followed by quenching using a quenching device. A longitudinal or transverse cross section of the mechanical component through the surface exhibits a hardness surface at the surface and a hardness H_(core) at the non-hardened core of the mechanical component (i.e. in the non-hardened base metal of the mechanical component. The hardness profile of the cross section (measured using the Vicker's Hardness Test or any other suitable method for example) exhibits a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the mechanical component and a second region between said first and third regions. The hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃. If a line is drawn on the hardness profile in the second region between the points

${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$

where 0<k<2 and k is a real number, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.

At least one part of the surface of such a non-through hardened mechanical component will exhibit increased surface hardness, increased wear resistance and/or increased fatigue and tensile strength. Furthermore, the induction hardening heat treatment used to produce such a mechanical component is more energy efficient and cost effective than a carburizing heat treatment and it has a shorter cycling time and provides better distortion control than a carburizing heat treatment. Furthermore, properties, such as the hardness, microstructure and residual stress, of the at least one part of the surface may be tailored as desired for a particular application.

According to an embodiment of the invention, k is 1.

According to an embodiment of the invention k is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.

According to an embodiment of the invention the hardness of the mechanical component in the second region determined along said line decreases by less than 30 HRC, less than 25 HRC, less than 20 HRC, less than 15 HRC or less than 10 HRC per mm.

According to another embodiment of the invention the hardness H_(surface) at the surface is between 55-75 HRC on the Rockwell scale, preferably between 58-63 HRC.

According to a further embodiment of the invention the hardness H_(core) at the non-hardened core of the mechanical component is between 15-30 HRC.

According to another embodiment of the invention the first region extends from the surface to a depth of up to 6 mm below the surface preferably to a depth of 1-4 mm below the surface, i.e. the material of increased hardness may extend to a depth of about 0.5-6 mm below the surface, preferably 1-2 mm below the surface.

According to a further embodiment of the invention the mechanical component may be a steel bar, a cylinder, a rod, a piston, a shaft, or a beam.

According to an embodiment of the invention the mechanical component may for example be used in automotive, wind, marine, metal producing or other machine applications which require high wear resistance and/or increased fatigue and tensile strength.

According to an embodiment of the invention, the mechanical component has a contact surface that allows a relative movement between the mechanical component and another component, e.g. a second mechanical component.

According to an embodiment of the invention, the contact surface comprises the at least one part that has been hardened.

According to an embodiment of the invention, the at least one part essentially corresponds to a contact surface.

According to a further embodiment of the invention the mechanical component comprises, or consists of a carbon or alloy steel with an equivalent carbon content of 0.40 to 1.10%, preferably a high carbon chromium steel. For example the mechanical component comprises/consists of 50CrMo4 steel having a composition in weight % 0.50 C, 0.25 Si, 0.70 Mn, 1.10 Cr, 0.20 P, 100Cr6 steel, or SAE 1070.

The present invention also concerns a method for surface hardening at least part of the interior or exterior surface of a mechanical component. The method comprises the steps of heating the at least one part of the surface with an electromagnetic induction coil to the ferrite/austenite transformation temperature or higher by induction heating, maintaining the at least one part of the surface at that temperature in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part, and quenching the at least one part of the surface in order to obtain a cross section of the mechanical component through the surface which exhibits a hardness H_(surface) at the surface and a hardness H_(core) at the non-hardened core of the mechanical component (i.e. in the non-hardened base metal of the mechanical component. The hardness profile of the cross section (measured using the Vicker's Hardness Test for example) exhibits a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the mechanical component and a second region between said first and third regions. The hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃. If a line is drawn on the hardness profile in the second region between the points

${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$

where 0<k<2 and k is a real number, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.

In conventional induction hardening, in which a mechanical component is heated up quickly and/or heat is now allowed to spread through the component, instable in-homogeneous austenite is formed. In the method according to the present invention, heat is allowed to spread through the mechanical component for a period of 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds or more so that stable, homogeneous austenite is formed. The expression “in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part,” is therefore intended to mean for a time period sufficient for stable, homogeneous austenite to form in the at least one part of the surface. By allowing homogeneous austenite to be formed, the formation of a transitional region, in which the hardness decreases steadily with depth below the surface rather than abruptly, is enabled/facilitated.

According to an embodiment of the method the hardness of the mechanical component in the second region determined along said line decreases by less than 30 HRC, less than 25 HRC, less than 20 HRC or less than 15 HRC per mm.

According to an embodiment of the method of the invention, k is 1. According to another embodiment of the invention k is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9.

It should be noted that the expression “an induction coil” as used throughout this document with reference to the mechanical component and method according to the present invention is intended to mean one or more induction coils. A plurality of induction coils operating in the same or a different manner, for example at the same or different frequencies, may for example be used to simultaneously or consecutively heat a plurality of parts of an exterior surface and/or an interior surface of a mechanical component, or one or more parts of a plurality of the mechanical components. The induction coil(s) may be arranged to surround one or more parts of a mechanical component that is to be hardened or the entire mechanical component.

In an embodiment of the method the induction coil is removed from the mechanical component, and a quenching device, such as a quench spray or ring, is used to immediately quench the at least one part of the surface that has been heat treated.

In another embodiment of the method the mechanical component is removed from the induction coil, and a quenching device, such as a quench spray or ring, is used to immediately quench the at least one part of the surface that has been heat treated.

According to another embodiment of the method the first region extends from the surface to a depth of up to 6 mm below the surface preferably to a depth of 1-4 mm below the surface.

According to a further embodiment of the invention the mechanical component may be a steel bar, a cylinder, a rod, a piston, a shaft, or a beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the schematic appended figures where;

FIGS. 1 & 2 show the steps of a method according to an embodiment of the present invention,

FIG. 3 shows a cross section of a mechanical component according to an embodiment of the present invention,

FIGS. 4 & 5 show hardness profiles of a mechanical component according to an embodiment of the present invention, and

FIG. 6 shows a comparison of hardness profiles obtained using carburization and induction hardening.

It should be noted that the drawings have not been drawn to scale and that the dimensions of certain features have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 schematically shows a thick-walled rotationally symmetrical mechanical component 10 manufactured from steel, namely a cylinder manufactured from a solid steel bar, which is to be used as a shaft, in cross section. The shaft 10 is, for example, 5 made of 50CrMo4 steel and comprises a hole 12 and a uniform circular cross section which extends all the way through the centre of the component in the longitudinal direction thereof. The shaft 10 has a minimum thickness T measured from an interior surface 12 a constituting the exterior perimeter of the hole 12 radially outwards to an exterior surface 10 a of the shaft 10 (or from 12 b to 10 b). The minimum thickness T is 7 mm, 10 mm, 20 10 mm, 30 mm or more. The hole 12 may alternatively be conical for example and thus have a non-uniform cross sectional size. Alternatively or additionally, the hole 12 may be arranged to have a non-uniform cross sectional shape.

An electromagnetic induction coil 14 is used to harden at least one part of the exterior 15 10 a, 10 b of the shaft 10. A source of high frequency electricity (about 1 kHz to 400 kHz) is used to drive a large alternating current through the induction coil 14. The relationship between operating frequency and current penetration depth and therefore hardness depth is inversely proportional, i.e. the lower the frequency the greater the hardness depth.

The passage of current through the induction coil 14 generates a very intense and rapidly changing magnetic field, and the part of the exterior surface 10 a, 10 b to be heated is placed within this intense alternating magnetic field. Eddy currents are generated within that part of the exterior surface 10 a, 10 b and resistance leads to Joule heating of the metal in that part of the exterior surface 10 a, 10 b. The exterior surface 10 a, 10 b of the shaft 10 is heated to the ferrite/austenite transformation temperature or higher by induction heating and preferably maintained at that temperature for 10-40 seconds.

In order to select the correct power supply it is first necessary to calculate the surface area of the shaft to be heated. Once this has been established empirical calculations, experience and/or a technique such as finite element analysis may be used to calculate the required power density, heating time and generator operating frequency.

In the illustrated embodiment the induction coil 14 is then removed and a quenching device 16, such as a quench spray or ring is used to immediately quenching the at least one part of the exterior surface 10 a, 10 b that has been heat treated. The at least one part of the exterior surface 10 a, 10 b may for example be quenched to room temperature (20-25° C.) or to 0° C. or less. The quenching device 16 is arranged to provide a water-, oil- or polymer-based quench to the heated exterior surface layer 10 a, 10 b whereupon a martensitic structure which is harder than the base metal of the shaft 10 is formed. The microstructure of the remainder of the shaft 10 remains essentially unaffected by the heat treatment and its physical properties will be those of the bar from which it was machined.

It should be noted that that a plurality of surfaces of the mechanical component according to the present invention may be surface hardened. For example, at least part of an interior surface 12 a, 12 b of the mechanical component may also be subjected to the method according to the present invention. The interior surface 12 a, 12 b may for example be hardened to a depth 1-2 mm while the exterior surface 10 a, 10 b of the mechanical component 10 may be hardened to a depth of 4-6 mm, depending on the application in which the mechanical component 10 is to be used.

As an example, a 60-200kW power supply, a frequency of 20-60 kHz, preferably 10-30 kHz or 15-20 kHz a total heating time of 10-40 seconds and a quenching rate and time of 2001/min and quenching time of 40-70 s respectively may be used to obtain a mechanical component according to the present invention.

FIG. 2 shows the position of the quenching device 16 while quenching is taking place. It should be noted that at least one other part of the outside surface 10 a, 10 b, 10 s of the shaft 10 may alternatively be subjected to another surface hardening heat treatment, such as induction hardening flame hardening or any other conventional heat treatment.

Furthermore, even though the shaft 10 in the illustrated embodiment has been shown in a horizontal position with the induction coil 14 and quenching device 16 being inserted horizontally, it should be noted that the shaft 10 may be oriented in any position. An induction coil 14 and quenching device 16 may for be moved vertically into place from the same or different ends of the roller 10. An induction coil 14 may for example be vertically lowered into place and a quenching device may be vertically raised as the induction coil 12 is withdrawn by raising it vertically.

FIG. 3 shows a longitudinal cross section of the shaft 10 after the heat treatment. Part microstructure produced by induction hardening using an electromagnetic induction coil 14 followed by immediate quenching using a quenching device 16.

The method according to the present invention results in the formation of a transition zone visible in both hardness and in microstructure. The heat treated part 18 of the exterior surface material 10 a, 10 b may namely have a hardness within the range of 55-75 HRC on the Rockwell, preferably 59-63 HRC. The material of increased hardness 18 may for example extend to a depth of up to 6 mm, preferably 1-4 mm below the exterior surface 10 a, 10 b measured radially downwards from the exterior surface 10 a, 10 b of the shaft 10 towards the interior surface of the shaft 12 a, 12 b respectively in the illustrated embodiment. Such a shaft 10 may be used for any application in which a part of the exterior surface 10 a, 10 b is subjected to increased wear, fatigue or tensile stress. Alternatively, the entire exterior surface 10 a, 10 b may be subjected to the method according to the present invention, depending on the application for which the shaft 10 is to be used.

The interior surface 12 a, 12 b of the shaft 10 may for example comprise a thread (not shown) arranged to mate with a corresponding thread of another component.

FIG. 4 shows a hardness profile 22 of a longitudinal cross section of a mechanical component according to an embodiment of the invention measured radially through a surface hardened region 18 in the direction of arrow 20 shown in FIG. 3. The hardness profile 22 exhibits a first region 24 whose hardness is substantially equal to the hardness H_(surface) at the outer surface 10 a, 10 b of the mechanical component 10, between 55-75 HRC, preferably between 58-63 HRC for example. The hardness profile 22 also comprises a third region 26 whose hardness is substantially equal to the hardness H_(core) at the non-hardened core 10 c of the mechanical component 10, between 15-30 HRC for example. The hardness profile 22 also comprises a second region 25 between the first region 24 and the third region 26 in which the hardness profile exhibits a smooth transition between the hardness of the first region 24 and the third region 26, i.e. the hardness profile exhibits a transitional region in which hardness decreases steadily with depth below the surface rather than abruptly. The hardness profile in the first region 24 has an average hardness Y₁, and the hardness profile in the third region 26 has an average hardness Y₃, and if a line is drawn on the hardness profile in the second region 25 between the points

${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$

where 0<k<2, the hardness of the mechanical component in the second region determined along the line decreases by less than 50 HRC per mm.

It should be noted that if the interior surface 12 a, 12 b of the shaft 10 is subjected to a method according to the present invention it will also have a similarly shaped hardness profile although its hardness values may be selected to be different from the hardness values of the exterior surface 10 a, 10 b of the shaft 10. The depth of the first region 24 and second region 25 may be chosen depending on the application in which the mechanical component 10 is to be used.

The dashed line in FIG. 4 shows a hardness profile 30 having a sharp demarcation between the hardness H_(surface) at the outer surface 10 a, 10 b of the mechanical component 10 and the hardness H_(core) at the non-hardened core 10 c of the mechanical component 10 in which the hardness decreases by more than 50 HRC per mm.

FIG. 5 shows a hardness profile 22 obtained using the method according to the present invention and determined from measured hardness values (measured using Vicker's Hardness Test or any other suitable method). The values may be extrapolated to a depth of 0 mm in order to obtain the hardness H_(surface) at the outer surface 10 a, 10 b of the mechanical component 10. In the illustrated embodiment the hardness of the mechanical component 10 at a depth of 6-8 mm below the surface of the mechanical component 10 may be considered to be the hardness H_(core) at the non-hardened core 10 c of the mechanical component 10.

FIG. 6 shows a comparison between hardness profiles obtained using carburization 24, conventional induction hardening 26 and the method according to the present invention 22. It can be seen that the hardness profile resulting from conventional induction hardening 26 decreases abruptly in the transitional region corresponding to the second region 25, as shown in FIG. 4. It can also be seen that the hardness profile obtained using the method according to the present invention 22 decreases much more steadily with depth.

Further modifications of the invention within the scope of the claims would be apparent to a skilled person. For example, rather than moving an induction coil and/or quenching device into position relative to a stationary mechanical component, a mechanical component may be moved into position relative to a stationary induction coil and/or quenching device. 

1. A mechanical component having a surface, wherein at least one part of said surface has been surface hardened by induction heating, whereby a cross section of the mechanical component through said surface exhibits a hardness H_(surface) at said surface and a hardness H_(core) at the non-hardened core of the mechanical component, wherein the hardness profile of said cross section exhibits: a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the mechanical component, and a second region between said first and third regions, wherein the hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃, and whereby if a line is drawn on the hardness profile in said second region between the points ${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$ where 0<k<2, the hardness of said mechanical component in said second region determined along said line decreases by less than 50 HRC per mm.
 2. The mechanical component according to claim 1, wherein said hardness of the mechanical component in said second region determined along said line decreases by one of: less than 30 HR per mm, less than 25 HRC per mm, less than 20 HRC per mm, and less than 15 HRC per mm.
 3. The mechanical component according to claim 1, wherein said hardness H_(surface) at said surface is between 55-75 HRC.
 4. The mechanical component according to claim 1, wherein said hardness H_(core) at the non-hardened core of the mechanical component is between 15-30 HRC.
 5. The mechanical component according to claim 4, characterized in that said first region extends from the surface to a depth of up to 6 mm below said surface.
 6. The mechanical component according to claim 1, wherein said mechanical component is fabricated into a form of at least one of a steel bar, a cylinder, a rod, a piston, a shaft, and a beam.
 7. A method for surface hardening at least part of the surface of a mechanical component, comprising steps of: heating said at least one part of the surface with an electromagnetic induction coil to the at least a ferrite/austenite transformation temperature by induction heating, maintaining said at least one part of said surface at least said ferrite/austenite transformation temperature in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part, and quenching said at least one part of the surface in order to obtain a cross section of the mechanical component through said surface which exhibits: a hardness H_(surface) at said surface and a hardness H_(core) at the non-hardened core of the mechanical component, whereby the hardness profile of said cross section exhibits: a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the mechanical component and a second region between said first and third regions, wherein the hardness profile in the first region has an average hardness Y₁, and the hardness profile in the third region has an average hardness Y₃, and whereby if a line is drawn on the hardness profile in the third region has an average hardness Y₃, and whereby if a line is drawn on the hardness profile in said second region between the points ${\frac{Y_{1} + Y_{3}}{2} + {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k\mspace{14mu} {and}\mspace{14mu} \frac{Y_{1} + Y_{3}}{2}} - {\left( \frac{Y_{1} - Y_{3}}{4} \right)*k}},$ where 0<k<2, the hardness of said mechanical component in said second region determined along said line decreases by less than 50 HRC per mm.
 8. The method according to claim 7, further comprises the steps of: heating said at least one part of the surface with an electromagnetic induction coil to at least the ferrite/austenite transformation temperature by induction heating, maintaining said at least one part of the interior surface at that temperature in order to allow for sufficient heat transport below the surface resulting in a sufficient austenitization of the at least one part, and quenching said at least one part of the surface in order to obtain a cross section of the mechanical component through said surface which exhibits a hardness H_(surface) at said surface and a hardness H_(core) at the non-hardened core of the mechanical component, and in which the hardness of the mechanical component in said second region determined along said line decreases by less than one of: less than 30 HRC per mm, less than 25 HRC per mm, less than 20 HRC per mm, and less than 15 HRC per mm.
 9. The method according to claim 7, further comprising a step of maintaining said at least one part of the interior surface at said temperature for at least one of 10 seconds, 20 seconds, 30 seconds, 40 seconds, 50 seconds, and greater than 50 seconds.
 10. The method according to claim 7, wherein the hardness profile of said cross section exhibits: a first region whose hardness is substantially equal to the hardness H_(surface) at said surface, a third region whose hardness is substantially equal to the hardness H_(core) at the non-hardened core of the mechanical component and a second region between said first and third region in which the hardness profile exhibits a smooth transition between the hardness of said first and third regions.
 11. The method according to claim 10, wherein said first region extends from the surface to a depth of up to 6 mm below said surface.
 12. The method according to claim 7, wherein said mechanical component is fabricated into a form of at least one of a steel bar, a cylinder, a rod, a piston, a shaft, and a beam.
 13. The mechanical component according to claim 1, wherein said hardness H_(surface) at said surface is between 58-63 HRC.
 14. The mechanical component according to claim 4, wherein said first region extends from said surface to a depth of between 1-4 mm below said surface.
 15. The method according to claim 10, wherein said first region extends from the surface to a depth from 2 through 4 mm below said surface. 